fix: Fix cases where both the chapter and part page counters are set

.changeset/clean-socks-think.md

@@ -0,0 +1,5 @@
+---
+"@vivliostyle/theme-base": patch
+---
+
+Fix cases where both the chapter and part page counters are set

packages/@vivliostyle/theme-base/css/common/meta-properties.css

@@ -68,4 +68,13 @@
    */
   --vs-document-root-counter-reset: ;
   --vs-first-page-counter-reset: ;
+
+  /**
+   * Counter increments
+   * It is used for incrementing CSS counters on a first page of a document
+   * Caution: Don't set counter-reset directly otherwise all other counters will be ignored.
+   * OK: :root { --vs-document-first-page-counter-increment: foo bar; }
+   * NG: @page :nth(1) { counter-increment: foo bar; }
+   */
+  --vs-document-first-page-counter-increment: ;
 }

packages/@vivliostyle/theme-base/css/partial/page.css

@@ -426,22 +426,37 @@
  * document counters
  */
 
+html.chapter,
+body.chapter {
+  page: chapter-document;
+}
+
 html.part,
-body.part,
-[role='doc-part'] {
-  page: part;
+body.part {
+  page: part-document;
+}
+
+html.part.chapter,
+body.part.chapter {
+  page: part-chapter-document;
 }
 
 @page :nth(1) {
-  counter-increment: vs-counter-doc;
+  counter-increment: var(--vs-page--doc-counter-increment, vs-counter-doc)
+    var(--vs-document-first-page-counter-increment,);
 }
 
-@page part:nth(1) {
-  counter-increment: vs-counter-doc vs-counter-part;
+@page chapter-document {
+  --vs-page--doc-counter-increment: vs-counter-doc vs-counter-chapter;
 }
 
-@page chapter:nth(1) {
-  counter-increment: vs-counter-doc vs-counter-chapter;
+@page part-document {
+  --vs-page--doc-counter-increment: vs-counter-doc vs-counter-part;
+}
+
+@page part-chapter-document {
+  --vs-page--doc-counter-increment: vs-counter-doc vs-counter-part
+    vs-counter-chapter;
 }
 
 /*
@@ -489,12 +504,6 @@ body.bibliography,
   page: bibliography;
 }
 
-html.chapter,
-body.chapter,
-[role='doc-chapter'] {
-  page: chapter;
-}
-
 html.conclusion,
 body.conclusion,
 [role='doc-conclusion'] {